Restore BitTorrent session asynchronously

Reduce the total startup time of the application and maintain sufficient responsiveness of the UI during startup due to the following:
1. Load resume data from disk asynchronously in separate thread;
2. Split handling of loaded resume data in chunks;
3. Reduce the number of emitting signals.

PR #16840.

src/base/torrentfileswatcher.h

@@ -78,6 +78,7 @@ private:
     explicit TorrentFilesWatcher(QObject *parent = nullptr);
     ~TorrentFilesWatcher() override;
 
+    void initWorker();
     void load();
     void loadLegacy();
     void store() const;